| Title | CATCHMENTS OF GROUNDWATER AND GEOTHERMAL SYSTEMS IN THE UPPER WAIKATO AREA, TAUPO VOLCANIC ZONE, NEW ZEALAND |
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| Authors | P.A. White, D. Graham, C. Tschritter |
| Year | 2015 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Water budgets, groundwater recharge, stream baseflow, Upper Waikato River, geothermal system recharge, Taupo Volcanic Zone |
| Abstract | Groundwater systems are important to the economy and ecology of the approximately 4400 km2 Upper Waikato River (UWR) catchment in the Taupo Volcanic Zone (TVZ), New Zealand. Cold groundwater is an important source of water for irrigation and industry and provides baseflow to rivers. Therefore, the groundwater systems are relevant to recreation and to the generation of a significant portion of New Zealand’s hydroelectric power generation. Geothermal reservoirs also provide a significant portion of New Zealand’s electrical power and also feed many unique geothermal features in the area. This paper describes a method to represent and characterise groundwater catchments in the Upper Waikato area, including geothermal systems. This representation was derived using: a 3D geological model of the area including basement, faults, aquifers and aquicludes; prolate spheroids to represent the location and volume of geothermal systems; water budgets (including groundwater flows with estimates of natural outflow from geothermal systems); a piezometric map of the UWR; and catchment boundaries defined by Waikato Regional Council (WRC). Groundwater catchments included: • the “shallow” cold groundwater systems that flow to surface water; • the “deep” geothermal fields, i.e., hot systems that are associated with geothermal fields; and • the “deep” cold groundwater systems that flow to geothermal fields. The location and volume of 14 geothermal fields in the UWR catchment were represented with prolate spheroids that intersect the ground surface. Each spheroid intersects the ground surface as an ellipse which represents the outer boundary of geothermal activity associated with each field. Geothermal fields in the UWR catchment are strongly associated with topographic lows suggesting that the piezometric elevation of cold groundwater systems is a factor in the distribution of the fields. For example, one major cluster of five fields (i.e., Reporoa, Waiotapu, Waikiti, Te Kopia and Orakeikorako) is located on the flanks of the Paeroa Range. The method was demonstrated for the Wairakei geothermal field. The “shallow” cold groundwater catchment of the Wairakei geothermal field extends to the west and includes WRC catchment 3039804. The western area provides an estimated 1.2 m3/s of recharge to the Wairakei geothermal field. This recharge contributes some, possibly all, of the estimated pre-development surface water discharge from the field of 1.2 m3/s, including geothermal fluid, and is consistent with near-isothermal conditions in some deep wells outside the field. Cold groundwater also flows into the Wairakei geothermal field from rainfall recharge within the geographic area of the field as demonstrated by cold water inflows to geysers and springs after development (White and Hunt, 2005). The field is probably bounded to the east by the Waikato River. Potential applications of the method include groundwater allocation. For example, groundwater allocation by WRC in the western area would be zero if they choose to protect all pre-development surface flows (i.e. cold and hot) from the geothermal field. |